There are currently numerous methods for preparing cutaneous tissue samples for diagnosis of various malignancies. In most instances, tissue samples are removed from the patient, packaged and then sent to a laboratory where tests are made, usually at a location remote from where the sample was taken. Thus, travel time for the sample between the operating room and the laboratory can delay an important diagnosis for several days or even weeks. Such an inordinate amount of time can be pivotal in the diagnosis and treatment of some of the more serious types of skin cancer.
By way of analogy, skin cancer develops much like the roots of a tree. These roots may extend horizontally, just beneath the surface, or they may invade the deeper tissues in a vertical pattern. Just as it is difficult to accurately determine the extent of its root system by studying the visible 5 portions of a tree, so it is with cutaneous carcinomas; small tumors may have deep roots, and very large tumors may have shallow, superficial roots.
Furthermore, the nature of a tree's root system is influenced by its genetic make-up, the soil in which it grows, and its supply of water and nutrition. With skin cancer, numerous factors of a similar nature effect its growth, such as location, blood supply, and the innate ability of the host to resist the growth of the tumor.
A number of treatment methods, including x-ray, surgery, electrosurgery and cryosurgery have been used effectively in treating skin cancers. However, these methods will fail if roots are left behind, thus allowing the tumor to return by regrowing at a later date. The problem is that the roots of a skin tumor are so small that they can only be seen with the aid of a microscope. Therefore, it is logical that a microscope be used to avoid leaving any of the cancer behind.
For these reasons, the Mohs technique was developed. The Mohs procedure allows the surgeon to excise a sample of cancerous tissue and systematically examine by microscope the undersurface of the margins by frozen sections, map the location of the residual tumor, and reexcise and reexamine subsequent layers of tissue until a cancer-free plane is reached. Since examination and diagnosis of the excised tissue are performed on site immediately after the sample is taken, it usually allows the patient to remain in the hospital or clinic until all cancerous tissue has been removed.
The Mohs technique is widely considered the most effective method for treating primary cutaneous malignancies and almost all recurrent, nonmetastic tumors. Because the surgeon can accurately identify tumor roots, or extensions, microscopically, residual clinically undetectable cancerous tissue can be removed, and normal tissue, ordinarily unnecessarily sacrificed by conventional surgery or irradiation, can be spared.
In the Mohs technique, cutaneous tissue specimens are removed by the surgeon in thin layers surrounding areas where a tumor is evident. The first micrographic layer that is removed includes all the clinically evident tumor plus a small peripheral margin of normal appearing tissue. The surgeon immediately diagrams the tissue specimen and applies various colored dyes to the edges of the tissue specimen for later orientation and identification.
The specimen is then placed upside down (epidermal or outermost surface down) onto a cryostat button. In this orientation, the deepest cutaneous layers will be the first layers sectioned, thereby theoretically allowing the lateral and deep margins of the tissue to be examined in the same plane under the microscope. As the specimen is placed on the button, the technician or user must continually attempt to flatten the lower or exposed surface of the specimen. If he or she is unsuccessful at flattening the lower surface, it will not be possible to section the specimen in such a way that an entire plane of tissue can be viewed through the microscope at one time. If an entire plane cannot be viewed simultaneously, there is a substantial risk that the sectioned specimen will provide inaccurate data and therefore that the surgeon or pathologist will be unable to properly diagnose the condition.
Once the tissue specimen has been properly placed on the button in an upside down orientation, it is frozen using standard cryostat microtome freezing techniques. The most common cryostat microtome freezing technique requires that the tissue specimen be surrounded by an imbedding medium such as OCT Compound. The specimen and embedding medium are then immediately saturated with a quick freezing compound such as Fisherbrand Histofeeze 2000. Other techniques involve freezing the specimen using an albumin solution and carbon dioxide. During the freezing procedure it is common for the technician to place the button upside down inside the cryostat on a device known generically as a cold plate. The cold plate holds the button and tissue specimen until the technician is ready to begin sectioning.
During the freezing procedure under current techniques, the tissue specimen is not held in place on the button and as a result has a tendency to move relative thereto. As indicated above, if such movement and the specimen is improperly placed on the button, there is a substantial risk that the sectioned specimen will provide inaccurate data and that the surgeon or pathologist will be unable to properly diagnose the condition.
Additionally, during the freezing procedure the sides of the tissue specimen have a tendency to droop towards the surface of the button. The technician must constantly use a scalpel or other device to lift the edges of the tissue specimen away from the surface of the button and attempt to maintain the tissue specimen in a flat orientation. If the technician cannot accomplish this two fold task during the freezing procedure, the specimen must be thawed and the whole freezing procedure must be repeated.